This invention relates to a belt conveying device, image forming apparatus equipped therewith, and adjustment method of belt skew controller in the belt conveying device.
In the belt conveying device, in which an endless belt is entrained about predetermined number of rollers, one of which runs the endless belt as a drive roller, there is a case that what is called, belt skew occurs, which is a phenomenon that a running endless belt moves in the width direction (a direction perpendicular to the belt running direction).
In the image forming apparatus, such as an inkjet printer for forming an image onto a recording medium, which is closely contacted with the endless belt as an object to be conveyed, and onto which an image is formed by jetting ink drops of respective colors onto the recording medium while the recording medium is conveyed, this belt skew phenomenon allows the recording medium to meander and causes relative position deviations of respective color images, which forms an inferior image.
Once an abnormal situation of the belt skew occurs, there is a problem that the endless belt meanders in one direction, comes into contact with the frame holding rollers and destroys the rollers.
Thus, in a past, unexamined Japanese Patent Application No. 09-169449 discloses a belt conveying device having a function for detecting and controlling the belt skew to correct the belt skew by detecting at least two values from the group of a belt skew amount, a belt skew deviation amount and a belt skew speed and correcting the belt skew. Unexamined Japanese Patent Application No. 10-231041 discloses a belt conveying device having a function for detecting and controlling the belt skew by detecting a belt skew speed and a belt skew position to correct the skew.
In a belt conveying device, in which an endless belt is entrained between a drive roller and a driven roller, one end of the rotation shaft of the driven roller in a longitudinal direction is fixed and the other end is arranged to be capable of oscillating in direction parallel to the conveying direction of the object to be conveyed, the movement direction in the width direction of the endless belt is determined by the inclination of the driven roller, and belt skew is corrected by oscillating the other end of the driven roller in the direction parallel to the conveying direction.
Here, in the belt conveying device, in which a weight roller, other than the drive roller and the driven roller, is provided in order to give a predetermined tension to the endless belt in the lower direction, in the case when the driven roller is oscillated, the weight roller moves in an up and down directions in response to the oscillation amount of the driven roller.
This operation will be described by referring to FIGS. 15(a) and 15(b). FIG. 15(a) illustrates a plan view of a belt conveying device and FIG. 15(b) illustrates a font view when viewing the belt conveying device from the conveying direction side, where numeral 100 denotes a drive roller, numeral 101 denotes a driven roller, numeral 102 denotes a weight roller and numeral 103 denotes an endless belt.
In case when the endless belt 103 shifts in the left direction in FIG. 15(a), one end of the driven roller 101 (the left edge in the Figure) is oscillated in the direction, in which the driven roller 101 moves away from the drive roller 100, and inclined in the direction, which is parallel with the conveyance direction of the endless belt 103 by a predetermined control value to correct this shift. In this situation, since the driven roller 101 is inclined, the tension applied to the left side of the endless belt 103 in the Figure is larger than that of right side of the endless belt 103.
In this case, since the weight roller 102, normally, moves in the direction for relieving the tension of the endless belt 103, the left side in FIG. 15(b) moves upward and leans to relieve the tension of the left side of the endless belt 103 as illustrated in FIG. 15(b). Based on this operation, the endless belt 103 becomes to be capable of moving in the reverse direction of the shift direction so as to correct the skew.
However, in the case when dirt or dust of the belt adhered onto the internal surface of the endless belt 103 is adhered onto the drive roller 100 and the friction coefficient of the drive roller 100 decreases thereby, even though the driven roller 101 is controlled with the same control value, the shift direction of the endless belt 103 is reversed.
This will be illustrated in FIG. 16. FIG. 16 illustrates a graph showing the relationship between the friction coefficient μ of the drive roller 100 and the shift direction of the endless belt 103.
As illustrated in FIG. 16, even though the oscillation of the driven roller 101 is controlled with the same control value, in case when the friction coefficient μ of the drive roller decreases, the inclination of the driven roller 101 and the shift direction of the endless belt 103 are reversed. Namely, the weight roller 102 is in a state as illustrated in a two-dot chain line, the endless belt 103 shifts in the left direction in FIG. 15(b). This is because the element for determining the shift direction of the endless belt 103 changes from the driven roller 101 to the inclination of the weight roller 102.
As described above, in the case the situation becomes to the state that the friction coefficient of the drive roller 100 decreases, there has been a problem that when detecting the belt movement amount of the endless bet 103 and oscillating the driven roller 101 based on the predetermined control value as it has been, the endless belt 103 moves in the opposite direction to that for which it was intended to originally correct.